The present invention relates to a spindle motor utilizing a dynamic pressure bearing for use in optical disks, magnetic disks and the like.
In recent years, optical and magnetic disk apparatuses have been developed to have a small size, a light weight, and a large capacity. In accordance with the popularization of notebook-type personal computers, spindle motors have been compelled to comply with the miniaturization and reduction in thickness of the computers, and have been demanded to have improved impact resistance and high precision. As a bearing for use in conventional spindle motors, a small-size ball bearing has been frequently used. However, when such a small-size ball bearing is used in accordance with the reduction in outer diameter of the spindle motors, a sufficient rotational accuracy cannot be obtained. The above has caused difficulty in achieving a large capacity, a significantly reduced impact resistance performance, and deterioration of the ball bearing, incurring a noise problem.
Lately, because the capacity increase cannot be achieved with the rotational accuracy of the ball bearing, a spindle motor utilizing a fluid dynamic pressure bearing filled with lubricating oil has been developed.
An exemplary construction of a spindle motor for use in a fixed magnetic disk drive (referred to as an HDD hereinafter) utilizing the conventional fluid dynamic pressure bearing will be described with reference to FIGS. 10 and 11. In FIGS. 10 and 11, there are shown a motor housing 31, a rotor hub section 32, a sleeve section 33, a shaft 34, and a thrust plate 35. There are further shown a magnet 36 fixed to the rotor hub section 32, a stator core 37, and a coil 38.
The motor housing 31 is provided with a cylindrical section 31a and a flange section 31b. The sleeve section 33 is mounted to an inner peripheral surface of the cylindrical section 31a, while a peripheral portion of the flange section 31b is mounted to a chassis of the HDD. Around the cylindrical section 31a is secured the stator core 37 around which the coil 38 is wound. The rotor hub section 32 is formed into a cup-like configuration with a disk receiving surface 32a and a disk inner diameter regulating cylindrical section 32b, and it rotates about the shaft 34, which is fixed to the center of the section 32b. To an inner peripheral portion of the cup-shaped rotor hub section 32 is secured the cylindrical magnet 36 that is circumferentially magnetized alternately with north poles and south poles.
The motor having the above construction is a radial type brushless motor. A current flows through the coil 38 to generate magnetic fields at salient poles of the stator core 37 and consequently generate a torque between the stator core 37 and the field-forming magnet 36 provided opposite to the stator core 37, thereby rotating the rotor hub section 32. By this operation, a magnetic disk (not shown) clamped on the rotor hub section 32 rotates.
Furthermore, a fluidic substance is filled inside the sleeve section 33 fixed to the cylindrical section 31a at the inner peripheral portion of the motor housing 31, while the thrust plate 35 is formed with spiral grooves. With this arrangement, the shaft 34 is rotatably supported in the direction of thrust by a dynamic pressure generated between the thrust plate 35 and the end surface of the shaft 34 according to the rotation of the shaft 34, and is also rotatably supported in the radial direction by a dynamic pressure generated at the fluidic substance in a non-contact manner with respect to the sleeve section 33.
Next, an outline of a manufacturing process of the thrust plate 35 will be described. A rod material (often made of stainless steels) is finished to a specified outer diameter, and then cut into sliced pieces. Each of the disk-shaped sliced pieces to be processed is designed to have a thickness greater by about 0.3 mm than the intended thickness. Then, the disk-shaped materials are each subjected to a heat treatment process to have an increased hardness and then to a lapping process to have an improved surface flatness through elimination of warp generated in the material due to the heat treatment process. Spiral grooves are formed on the lapped surface by etching.
However, according to the above-mentioned prior art spindle motor, for the manufacturing of the thrust plate 35, the material pieces are positioned one by one for the etching process, and therefore, a seriously degraded manufacturing efficiency results. Furthermore, since the material pieces are processed one by one, a large variation in depth of the spiral grooves occurs in the etching process. Therefore, it is required to perform again the lapping process while measuring the groove depth, and this has further degraded the manufacturing efficiency.
For the above reasons, the thrust plate 35 provided with the spiral grooves to be used as the dynamic pressure bearing in the direction of thrust within the dynamic pressure bearing structure becomes expensive because of the manufacturing method, and this also pushes up the cost of the spindle motor.